A method has been developed for the estimation of local rates of protein synthesis in brain in vivo. The method is based on the use of L-(14C)leucine as a tracer for the incorporation of leucine into protein. Six kinetic models for the behavior of leucine in brain have been designed. By mathematical analysis of the kinetics of exchange of the amino acid between plasma and the tissue pool(s) and its incorporation into protein, equations have been derived for each model that define the rate of amino acid incorporation into protein in terms of the time course of plasma- specific activity, final tissue concentration of 14C, and experimentally determined kinetic constants. Tissue concentrations of 14C are determined by quantitative autoradiography. Experiments have been carried out to test the validity of the various models and to determine the kinetic constants to be used in the operational equation. Studies of the effects of normal development, sleep, anesthesia, electrical stimulation and hypothyroidism on cerebral protein synthesis have been undertaken in order to examine the potential usefulness of the leucine method.